Ripple reduction in a half-lattice crystal filter using three paralleled crystals resonant at lower,center and upper edge of pass-band



Aprll 7, 1970 R. G. WRENCH ETAL 3,505,617

RIPPLE REDUCTION IN A HALF-LATTICE CRYSTAL FILTER USING THREE PAHALLELEDCRYSTALS RESONANT AT LOWER, CENTER AND UPPER EDGE OF PASSBAND Filed Feb.12. 1968 l7 /5 m g R ATTENUATION APPROX. 3d!) RIPPLE MlNlMlZED RIPPLEFREQUENCY F/G. Z

INVENTORS RICHARD G. WRENCH RICHARD H. TUZN/K United States Patent3,505,617 RIPPLE REDUCTION IN A HALF-LATTICE CRYS- TAL FILTER USINGTHREE PARALLELED CRYS- TALS RESONANT AT LOWER, CENTER AND UP- PER EDGEOF PASS-BAND Richard G. Wrench, Auburn, Wash., and Richard H. T 111-nik, Erie, Pa., assignors, by mesne assignments, to the United States ofAmerica as represented by the Secretary of the Navy Filed Feb. 12, 1968,Ser. No. 704,744 Int. Cl. H03h 9/00 U.S. Cl. 33372 1 Claim ABSTRACT OFTHE DISCLOSURE In filters, a half-lattice section with three crystalsare paralleled and are resonant, respectively, to i The passband isbroadened and squared, and ripple is minimized.

BACKGROUND In the classic lattice filter section, impedance Z and 2,,are connected in series with and in shunt across the circuits throughthe filter and may have two, three or more poles and zeros in thepassband. The impedances Z,, and Z customarily comprise resonant andanti-resonant circuits. More recently the Zs have been replaced bypiezoelectric crystal elements, and in the interest of economy, the fulllattice design is simplified by the so-called halflattice design. Here,the reactances of one LC resonator is doubled and the reactances of thesecond LC resonator is halved, whereupon two arms of the full latticemay be omitted.

Unfortunately, the impedance of the filter thus constituted in thepassband is not smooth but permits as much as three db ripple variationin the output.

The object of this invention is to provide an improved half-latticefilter with minimum ripple in the passband of the filter.

SUMMARY The object of this invention is attained by connecting threepiezoelectric elements, A, B, and C in parallel and to one inputterminal. The other terminals of crystal elements A and B are connectedto one end of an antiresonant circuit and the other terminal of crystalelement C is connected to the other end of the anti-resonant circuit.The electrical center of the anti-resonant circuit is grounded and theoutput terminals are connected to the upper end of the anti-resonantcircuit.

Other objects and features of this invention will become apparent tothose skilled in the art by referring to the preferred embodimentdescribed in the following specification and shown in the accompanyingdrawing in which:

FIG. 1 is a schematic circuit diagram of the filter of this invention;and

FIG. 2 is a diagram of the attenuation-frequency characteristic of thefilter of FIG. 1.

The four terminals of the filter of this invention are indicated in FIG.1 at 11, 12, 13, and 14. The signal source will be assumed to beconnected to the left hand terminals 11 and 12 and is indicated at 10.The signal source, may comprise a narrow or a broad band of frequencies.The load connected to the other terminals of the filter is shown at 15,is resistive, and is matched impedice ance-wise with the filter. Forconvenience terminals 12 and 14 will be referred to as a referenceground.

The inductive reactance 16 is connected at one end to the outputterminal 13. Capacitive reactance 17 is connected across the winding andis of such a value as to cause the tank circuit to resonate in thepassband. The electrical center of the resonant circuit is grounded,preferably by a mid tap or center tap to the winding 16. Inductances Lon either side of the tap are approximately equal. Alternatively,capacitance 17 could comprise two series condensers with the commonterminal grounded.

According to an important characteristic of this invention threepiezoelectric crystal elements, A, B, and C are employed. Crystals A andB are connected in parallel and between the input terminal 11 and outputterminal 13 and hence to the upper end of anti-resonant circuit 16-17,while the third crystal C is connected between terminal 11 and theopposite on lower end of the anti-resonant circuit 1617. The nextimportant feature of this invention the crystal C is resonant to thecenter frequency f of the passband of the filter while crystals A and Bare, respectively, resonant to frequencies f0+ and Julwhere A1 is thedesired bandwidth of the filter.

The attenuation characteristic shown in solid line at FIG. 2 illustratesthe distinct three pole passband attenuation of the conventional fulllattice filter where two crystals resonant at the same frequency areconnected in the filter. Such a ripple is substantial, and is found toapproximate three db and to produce objectionable noise in the output ofthe filter. When, however, the two crystals A and B are, respectively,slightly above and below the resonant frequency f the ripple is markedlyreduced as shown by the dotted line in FIG. 2. That is, by slightlystaggering the resonant frequencies of the three crystals, according tothis invention, the passband portion of the attenuation curve issmoothed to minimize ripple. The squared sides of the curve are notdeteriorated. The minimized ripple within the passband can possibly beattributed to the detuning and the slightly decreased Q of the filtercircuits with the increase in apparent inherent resistance of thecrystal elements.

The circuit parameter values depends upon the center frequency,bandwidth, and impedance level of the circuit. The resonant frequency ofcrystal C is the center frequency f of the filter while the resonantfrequency of crystals A and B are disp'aced /2 the bandwidth from thecenter frequency. Typical values for f are 450 kc. and of A is 3 kc. Thecenter-tapped winding is necessary to construct the hybrid half-latticecircuit, but the values of the winding design are not critical providedthe winding is tune'able to the center freqency f The transformer designmay be of standard toroidal type, and the capacitance value is thatrequired to tune the transformer. The attenuation versus frequency curvehas a nominal 40 db to 3 db selectivity of 3.5 to 1.

What is claimed is:

1. A half-lattice four-terminal crystal filter for coupling a signalsource to a load, said filter comprising:

three piezoelectric crystal elements A, B, and C,

a parallel resonant inductance-capacitance circuit connected at one endto one output terminal, the midpoint of said inductance being connectedto the other output terminal,

crystal element C being connected between the other end of said resonantcircuit and one terminal of said source, crystal elements A and B beingconnected in parallel between said one source terminal and said oneoutput terminal,

crystal element C being resonant to the center fre- 2,990,525 6/1961Grant 333-72 quency, f of the passband, A), of said filter and 3,359,51212/ 1967 Malinowski et 211. 333-72 elements A and -B resonant,respectively, to 3,416,104 12/ 1968 Argoudelis 33372 A A OTHER RFERENCES f0+ and f I" E 2 2 0 Kosowsky, D. I.: High Frequency CrystalFilter De- References Cited sign Techniques and Appllcanons, Proc. I. R.E., February 1958, vol. 46, No. 2, pp. 419-429. UNITED STATES PATENTS2,097,458 11/1937 Hansen 333 72 10 HERMAN KARL SAALBACH, PrimaryExaminer 2,266,658 12/ 1941 Robinson 333-42 W. H. PUNTER, AssistantExaminer 2,959,752 11/1960 Kosowsky 333-72

